PVC layer for use in a photoelectrochemical cell

ARTICLE IN PRESS

Solar Energy Materials & Solar Cells 91 (2007) 740–744 www.elsevier.com/locate/solmat

A novel TiO2/PVC layer for use in a photoelectrochemical cell F. Touatia,b, J.F. Cassidya,, K.G. McGuiganb a

School of Chemical and Pharmaceutical and Sciences, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland Department of Physiology and Medical Physics, Royal College of Surgeons of Ireland, 123 St Stephens Green, Dublin 2, Ireland

b

Received 28 August 2006; received in revised form 4 January 2007; accepted 4 January 2007 Available online 21 February 2007

Abstract Titanium dioxide (TiO2) has been widely used with UV light to degrade organic waste contaminants. Immobilised layers of TiO2 on electrode surfaces have shown enhanced activity when appropriate potentials have been applied. In this work, it is shown that a novel immobilised layer of TiO2 on an electrode, a TiO2/poly(vinylchloride) composite cast from THF, mineralises acetone or starch when exposed to a xenon arc light only if the electrode is connected to a Pt electrode where concomitant reduction of oxygen occurs. When an isolated electrode with an immobilised TiO2 layer is exposed to UV light in a solution of starch or acetone, no decrease in acetone or starch concentration is observed. r 2007 Elsevier B.V. All rights reserved. Keywords: Fuel cell; TiO2; Photoelectrochemistry

1. Introduction In an effort to remediate wastewater, there has been, inter alia, photochemical methods and electrochemical methods based on microbial fuel cells that are used to reduce the chemical oxygen demand (COD). UV and solar radiation have been used with or without titanium dioxide (TiO2) to degrade organic compounds in aqueous solutions [1–4]. Immobilised TiO2 layers have been employed as catalysts for the oxidation of contaminants in wastewater [5,6] including for example paraquat [7]. Sensitised layers have been employed which have enhanced the process by using visible light [8]. Immobilised TiO2 layers have been also employed for bacterial inactivation [9,10]. Immobilised TiO2 layers have also been used on electrodes where potentials were applied which enhanced the catalytic process. The application of this potential reduces electron hole recombination [11–13]. Organic waste has the potential to yield energy with a typical carbohydrate oxidation represented as follows [14]: fCH2 Og þ O2 ! CO2 þ H2 O;

DG ¼ 475 kJ mol1 ,

Corresponding author. Tel.: 353 1 402 4779; fax: 353 1 402 4989.

E-mail address: [email protected] (J.F. Cassidy). 0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.01.002

and as such there is energy available from a thermodynamic basis. Photocatalytic methods or microbial fuel cells have been used to catalyse this process. Microbial fuel cells are cells composed of a two electrode configuration with an anode where organic substrates are oxidised while oxygen is reduced at the cathode, frequently a precious metal [17]. These have been employed in a one compartment cell with domestic wastewater as substrate [15] or food processing wastewater [16] along with swine waste water [17] or cysteine solutions [18]. The purpose of this paper is to demonstrate that a TiO2/ poly(vinyl chloride), (TiO2/PVC) composite, which is easy to cast on a glassy carbon electrode, can be successfully used as an anode in a photoelectrochemical cells. Here the cell is used to degrade organic compounds as has been done elsewhere [19,20]. 2. Experimental The light source was a 150 W Xenon arc lamp (150 W/1 XBO, Osram, Germany) fitted with a rear reflector and UV grade fused silica F1 collecting optics. The anode consisted of a glassy carbon sheet (Tokai) of area 3.8 cm2. The cathode was a Pt sheet (area ¼ 1 cm2). For the TiO2/PVC electrode (a composite coated on the glassy carbon

ARTICLE IN PRESS F. Touati et al. / Solar Energy Materials & Solar Cells 91 (2007) 740–744

working electrode), the layer was prepared with a suspension of TiO2 (Degussa P25, 2.090 g/dm3) and 0.7 g/dm3 polyvinylchloride (PVC) in 10 mL tetrahydrofuran, THF. This suspension was sonicated for 10 min and 0.2 mL of this suspension was coated on a glassy carbon electrode (area ¼ 3.8 cm2) and the modified electrode allowed to dry at room temperature for 15 min. The fuel cell was a two compartment cell (each compartment holding approximately 50 mL), separated by a glass frit. The current transient was measured for 100 ppm acetone in 0.1 M KCl in a fuel cell consisting of a carbon electrode coated with a TiO2/PVC composite and a Pt cathode. The current was passed directly through a resistance (1 kO) and the voltage fed to a Recorder lab YT Chart recorder. Acetone 100 ppm in 0.1 M KCl was degraded in a fuel cell with carbon electrode (3.8 cm2) coated with PVC/TiO2 as an anode and platinum electrode (area ¼ 1 cm2) as cathode. The anode was exposed to the 150 W xenon lamp for 4 h. Every half an hour a 1 mL sample from the electrolyte solution was sampled. Gas chromatography was carried out on Shimadzu GC-8 with a 10% Carbowax 1500 (80–100 mesh) packed column and fitted with a FID detector. Before injection onto the GC column 400 mL of the sample was mixed with 100 mL isobutanol (500 ppm) as internal standard. From this mixture, 1 mL of the mixture was injected directly in GC column. Areas were calculated on a Shimadzu CR8A Chromatopac integrator. A calibration plot was used to determine the decrease in acetone concentration A standard stock starch solution (500 ppm) was prepared by dissolving 50 mg of starch powder in approximately 50 mL 0.1 M KCl solution. The solution was boiled until the starch completely dissolved. This was allowed to cool and was placed in a 100 mL volumetric flask and the volume was made to the mark with 0.1 M KCl. 0.2, 0.4, 0.6, 0.8 and 1.0 mL aliquots of standard starch solution containing the respective quantity of starch 100, 200, 300, 400, 500 ppm were made separately to a total volume of 1 mL with distilled water and mixed with 1.2 mL 2 M acetic acid, 0.25 mL 10% of potassium iodide and 2.5 ml of M/600 potassium iodate. The absorbance of each solution was measured with at 570 nm against the blank solution containing 1 mL water in place of the starch solution mixed with the same reagents added to samples. Twenty five millilitres of starch solution (300 ppm) was placed in a two compartment cell consisting of platinum electrode as cathode and 3.8 cm2 glassy carbon coated with the TiO2/PVC composite as anode. This cell was exposed to 150 W xenon lamp for 120 min. A 1 mL sample was taken every 30 min. Each photolyte sample was treated with the reagent as above (1 mL sample+1.2 mL 2 M acetic acid, 0.25 mL 10% of potassium iodide and 2.5 mL of M/ 600 potassium iodate), before measuring the absorbance at 570 nm against the water reagent blank (1 mL distilled water+1.2 mL acetic acid+0.25 mL KI+2.5 mL KIO3).

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Spectra were collected on a Shimadzu UV 16 instrument. The decrease in starch concentration was determined by using a Beers Law plot. 3. Results and discussion Previously it has been shown that TiO2 suspensions have successfully degraded isopropanol in the presence of UV light. Indeed we have shown that there is an increase in the concentration of acetone as the concentration of isopropanol decreases [10]. In the system described here, the fuel cell configuration allows the mineralisation of both isopropanol and acetone. The cell comprises a two compartment cell with a glassy carbon anode coated with a TiO2/PVC composite. The cathode is a Pt foil of area 1 cm2. The operation of the fuel cell arises from the indirect ‘combustion’ of acetone. This combustion is catalysed by the exposure of the TiO2 coated anode to UV light, with a concomitant O2 reduction at Pt. Fig. 1 shows the effect of 150 W xenon arc light exposure on the current of a cell containing 100 ppm acetone, in 0.1 M KCl, in the anode compartment of a two compartment fuel cell. The anode consists of a glassy carbon plate coated with a TiO2/ PVC composite and the cathode is a Pt electrode (area ¼ 1 cm2). It can be seen that there is an increase in current on exposure to UV light. Such transients have been published elsewhere and current transients have been recorded using a light of lower energy (a 60 W tungsten lamp) with formic acid as a substrate [21]. The current transient in

i 10µA

OFF

40 s

ON 0

Time Fig. 1. Current transient of a fuel cell consisting of glassy carbon (3.8 cm2) coated with PVC/TiO2 composite as anode and a Pt electrode in 0.1 M KCl in a separate compartment as cathode. The solution on the anode side is 100 ppm acetone in 0.1 M KCl. The cell is in ambient light and the transient occurs when the cell is exposed to the xenon arc lamp.

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Fig. 1 was determined by collecting the voltage change over a 1 kO resistance connected between the two electrodes. A resistance of magnitude 1 kO has typically been used in previous work [17]. The increase in current indicates that spontaneous photoelectrochemistry is taking place and that the anode is behaving like other electrodes which have been calcined at 450 1C [20]. The increase in current is not as great as has been seen for other substrates such as formic acid since acetone is not as readily oxidised [21,22]. Fig. 2 shows photoelectrodegradation of 100 ppm acetone solution where the TiO2/PVC coated anode is exposed to light and linked through a 1 kO resistance to a cathode in the cell. It can clearly be seen that there is decrease in the acetone concentration with time. In the

same figure is seen another response (’), which was as a result of the same experiment without connection of the two electrodes. In the latter case, there was no decrease in the acetone concentration during the exposure time. There is a zero-order decrease in acetone concentration as has been seen before in photodegradation of isopropanol and formic acid using a similar electrochemical configuration [12,22]. The zero-order decrease in acetone concentration is due to the limited available surface area of the TiO2. It can be seen in Fig. 2 that the anode, which is easily fabricated can be used to degrade acetone. Rather than using acetone as the substrate that has a simple electrochemistry, starch was used as a more complex substrate. Fig. 3 shows that for a solution of starch in

acetone concentration (ppm)

120 110

y = -0.0048x + 101.97 R2 = 0.0412

100 90 80 70 60

y = -0.1546x + 108.99 R2 = 0.9658

50 40

0

100

200

300

400

Exposure time (min)

Fig. 2. Photoelectrodegradation of 100 ppm acetone in a two compartment cell. The anode is glassy carbon (3.8 cm2) coated with TiO2/PVC; the cathode is 1 cm2 Pt electrode. The anode was exposed to the 150 W xenon arc lamp. (E) Two electrodes in the cell connected, (’) two electrodes in cell unconnected.

350

y = -0.0506x + 299.4 R2 = 0.8913

starch consentration (ppm)

300 250

y = -1.0131x + 313.63 R2 =0.959

200 150 100 50 0 0

50

100

150

200

250

exposure time (min) Fig. 3. Photoelectrodegradation of 300 ppm starch in 0.1 M KCl solution exposed to 150 W xenon lamp (’) when the glassy carbon electrode connected to the platinum electrode in the fuel cell (E) lamp exposed to the anode (glassy carbon electrode) without being connected to the cathode in the two compartment cell.

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C3 H8 O þ 4:5 O2 ¼¼¼¼¼ 4H2 O þ 3 CO2

OFF

ON -0.50

POTENTIAL/V

0.1 M KCl, there is a decrease in concentration when the cell is connected and no change in concentration when the cell is not connected. It can be seen that on exposure to UV light alone, that there is no change in the starch concentration as is the case for acetone in Fig. 2. It is only when the illuminated anode is connected to the platinum that complete mineralisation of starch and acetone occurs. Since no mineralisation occurs for acetone in an unconnected cell, there is recombination of the electron hole pairs before the holes oxidise the acetone and the electrons reduce O2. In the case of the isopropanol reaction

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-0.30

-0.10

20 sec.

DG ¼ 1957 kJ mol1 whereas for acetone C3 H6 O þ 4 O2 ¼¼¼¼ 3H2 O þ 3 CO2 DG ¼ 1740 kJ mol1 : When the cell is connected, acetone is oxidised by the holes while the electrons pass to the Pt allowing oxygen to be reduced more readily. Since there is no great difference in the free energies associated with the oxidation of isopropanol or acetone, the ease with which they oxidise is due to slow kinetics. Isopropanol is oxidised easily by holes at the TiO2 allowing oxygen to be reduced. However, acetone is more slowly oxidised by holes accompanied by a relatively slow oxygen reduction by electrons at the TiO2. This process is accelerated by allowing the oxygen to be reduced at a Platinum electrode which facilitates the acetone oxidation. The cell when connected does yield a current under ambient conditions; however even though this current is approximately half that when the anode is exposed to UV light, this process does not contribute to a decrease in the concentration of acetone. For formic acid, it was found that the increase in current, when the anode was exposed to UV light, was much greater than in this work for starch or acetone. Previous work on the examination of kinetics of the oxidation of organic compounds at polycrystalline TiO2 showed that formic acid is primarily oxidised directly by photogenerated holes, while methanol oxidation occurs indirectly through surface bound hydroxyl radicals. The process was accelerated by applying a positive voltage to reduce electron hole recombination [23]. Along with formic acid, acetate and oxalate are easily oxidised when positive potentials are applied to the electrode [24]. In the cell described here, the driving force is limited by the kinetics of the slower process of the oxidation of the organic substrate or the reduction of O2. There are two models that can be used to characterise the PVC/TiO2 composite. Model A consists of a layer of electrically isolated TiO2 particles suspended in a porous PVC matrix permeable to solvent and electrolyte. On exposure to light, oxidation of acetone occurs at individual TiO2 particles along with a simultaneous reduction of

TIME Fig. 4. Open circuit potential transient for a carbon disk (diameter ¼ 3 mm) coated with a PVC/TiO2 composite. Potentials were measured with respect to SCE in a one compartment cell and the solution was 0.01 M formic acid in 0.1 M KCl, equilibrated with air. A 150 W xenon lamp was the light source for the transient.

oxygen forming H2O2 which diffuses to the underlying carbon for oxidation. Model B consists of a compact TiO2 layer glued together with PVC. At a layer close to the electrode, where the TiO2 particles are electrically linked to the anode, acetone is oxidised by the photogenerated hole and the electron passes directly to the underlying carbon. It is more likely that model B applies here since the oxygen reduction is four electron leading to OH. Further support for the model B is seen in Fig. 4 where a potential transient at a TiO2/PVC composite electrode is similar to those in the literature for TiO2 coatings electrically linked to the underlying electrode. Examples are layers formed from titanium (IV) n-butoxide [25] or annealed TiO2 layers on indium tin oxide [12]. It can be seen that although there is considerable energy available for the degradation of the acetone from the UV light, this degradation does not occur to any extent (even though there is some current flowing before exposure to light in Fig. 1). The increase in efficiency of the degradation is due to the increased rate of oxygen reduction at the platinum cathode in a connected cell than at the TiO2 nanoparticle in the unconnected cell. Further work will be done relating the charge passed and the change in the acetone concentration. Kaneko [20] has recently proposed a ‘photofuel’ cell based on a photoanode and a platinised Pt cathode using a 500 W xenon lamp. Various model compounds such as methanol, glucose, urea and amino acids were used as model biomass materials and a maximum of 78% of the energy of the fuel was converted to electrons. The cell was similar in that it used nanoporous TiO2 and UV light with oxygen reduction occurring at Pt.

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4. Conclusion In this paper, it is demonstrated that a TiO2/PVC composite which is easily cast on an electrode can exhibit photoelectrochemistry resulting in the decrease in the concentration of acetone or starch. There is a mineralisation of compounds that are not easily degraded by UV/TiO2 systems alone by this photoelectrochemical fuel cell. Such a system can be used to enhance the effectiveness of (expensive) UV light in the degradation of organic components without the requirement to supply any external energy. Such a system can be extended to solar light through the use of screen printed electrodes based on carbon ink. Acknowledgements JC acknowledges a Team Research Scheme grant from DIT. FT acknowledges a grant from the Government of Libya. References [1] S. Malato, J. Blanco, A. Vidal, C. Richter, Appl. Catal. B. 37 (2002) 1. [2] O.M. Alfano, D. Bahnemann, A.E. Cassano, R. Dillert, R. Goslich, Catal. Today 58 (2000) 199. [3] D. Bahnemann, Sol. Energy 77 (2004) 445. [4] S. Devipriya, S. Yesodharan, Sol. Energy Mater. Sol. Cells 86 (2005) 309. [5] J.A. Byrne, B.R. Eggins, N.M.D. Brown, B. McKinney, M. Rouse, Appl. Catal. 17 (1998) 25.

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